1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbons (HC).
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NOx, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as Pt, Pd and/or Rh for reducing NOx to N2 and for oxidizing CO and HC to CO2 and H2O. In this way, the three-way catalytic converter works as a catalyst both for oxidation and reduction.
Various researches have been made to improve the performance of a three-way catalytic converter. One of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO2) which has an oxygen-storing capacity (OSC); that is, the capacity to occlude gaseous oxygen in the crystalline structure and to release the occluded oxygen from the crystalline structure. More specifically, CeO2 is added to a three-way catalytic converter for adjusting the oxygen concentration of gaseous atmosphere, so that excess oxygen in the gaseous atmosphere is occluded into the crystalline structure of CeO2 in an oxygen-rich state for assisting the catalytic converter in reducing NOx to N2 while releasing the occluded oxygen into the gaseous atmosphere in a CO- and/or HC-rich state for assisting the catalytic converter in oxidizing CO and HC to CO2 and H2O.
Japanese Patent Publication 5-47263 (which is the granted version of JP-A-63-156545) discloses a catalytic converter for cleaning exhaust gas wherein fine particles of zirconia (ZrO2) carrying a precious metal (e.g. Pt, Rh) are coated on a heat-resistant honeycomb support together with particles of heat-resistant inorganic oxide (e.g. alumina) and particles of an oxygen-storing oxide of a rare earth element. In such a converter, the heat-resistant inorganic oxide and the rare earth element oxide intervene between the agglomerates of the zirconia particles for preventing the zirconia particle agglomerate from growing due to agglomerate-to-agglomerate sintering in high-temperature oxidizing atmosphere, thereby limiting a decrease of specific surface area which may result in degradation of catalytic activity.
While the prior art catalytic converter described above prevents sintering between the zirconia particle agglomerates, it fails to prevent the zirconia particles themselves from growing due to particle-to-particle sintering. Further, depending on its mounting position, the catalytic converter may be subjected to an extremely high temperature which prompts the grain growth of zirconia.
More specifically, there is an increasing demand for shifting the mounting location of the catalytic converter from below the body floor to the exhaust manifold which is near the engine, whereby the catalyst can be quickly warmed up after starting the engine. However, when the catalytic converter is located near the engine, it may be of ten exposed to a high temperature of no less than 900xc2x0 C. (or sometimes even higher than 1,000xc2x0 C.), which may cause grain growth of ZrO2 due to particle-to-particle sintering. As a result, the specific surface area of ZrO2 reduces to result in a decrease of the catalytic activity of the precious metal carried on the zirconia particles.
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which does not result in an excessive decrease of the catalytic activity of a precious metal or metals even under severe operating conditions above 900xc2x0 C.
According to the present invention , there is provided a1. A catalytic converter for cleaning exhaust gas comprising: a heat-resistant support; particles of a zirconium complex oxide of the following formula,
Zr1xe2x88x92(x+y)CexRyO2xe2x88x92z
where xe2x80x9cRxe2x80x9d represents at least one element selected from a group consisting of Al and rare earth elements other than Ce, xe2x80x9czxe2x80x9d represents the degree of oxygen deficiency determined by the valence and content of the contained Al and/or rare earth element, 0.1xe2x89xa6x+yxe2x89xa60.5, 0.1xe2x89xa6xxe2x89xa60.5, and 0xe2x89xa6yxe2x89xa60.2; a combination of Pt and Rh coexistently carried on the zirconium complex oxide particles; and particles of an oxygen-storing complex oxide of a rare earth element; wherein the zirconium complex oxide particles are coated on the heat-resistant support together with the oxygen-storing complex oxide particles.
The present invention features that a part of zirconium in zirconia (ZrO2) is substituted with cerium (Ce), and optionally with aluminum (Al) and/or a rare earth element or elements other than cerium. Such substitution restrains the mass transfer of zirconium at high temperature, thereby preventing the zirconia particles from unduly growing. As a result, Pt and Rh coexistently carried on the zirconia particles can retain their catalytic activity above a predetermined level even at high temperature.
Preferably, at least a part of the zirconium complex oxide may be solid solution. This feature is additionally effective for restraining the mass transfer of Zr, thereby enhancing the durability of the catalytic converter at high temperature.
The precious metals Pt and Rh are coexistently carried on the zirconium complex oxide particles for the following reason. If Pt alone is carried on the zirconium complex oxide particles, the particles of Pt exhibit a tendency to grow due to the mass transfer of Pt at high temperature. By contrast, if Rh coexists, it restrains the mass transfer of Pt to prevent grain growth (presumably due to the formation of a rhodium oxide layer on the Pt particles which restrains the mass transfer of Pt).
In the above formula, the relation xe2x80x9c0.1xe2x89xa6x+yxe2x89xa60.5xe2x80x9d needs to be met because if the ratio of substitution of zirconium with cerium and other elements is lower or higher than this range, it becomes difficult to effectively prevent the mass transfer of zirconium. If the substitution ratio is higher than this range, Pt and Rh coexistently carried on the zirconium complex oxide may adversely interact with each other to lower the catalytic activity. The same reasons also apply to xe2x80x9c0.1xe2x89xa6xxe2x89xa60.5xe2x80x9d . Preferably, the value of the xe2x80x9cx+yxe2x80x9d should lie in the range of 0.2xcx9c0.3, whereas the value of the xe2x80x9cxxe2x80x9d should be set in the range of 0.1xcx9c0.28. It should be understood that the content of Zr in the zirconium complex oxide may include 1xcx9c3% of hafnium (Hf) which is inevitably contained in Zr ores.
Examples of rare earth elements xe2x80x9cRxe2x80x9d other than Ce include Y, Sc, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Of these examples, La and Nd are preferred. The reason for partially substituting Zr of the zirconium complex oxide with the rare earth element other than Ce is that such substitution stabilizes the uniform fluorite structure of the zirconium complex oxide at room temperature while also restraining the mass transfer of Zr in cooperation with Ce.
Alternatively or additionally, a part of Zr in the zirconium complex oxide may be substituted with Al alone or in combination with Y.
The value of the xe2x80x9cyxe2x80x9d in the above formula is 0xcx9c0.2. In this way, the value of the xe2x80x9cyxe2x80x9d may be 0 so that the zirconium complex oxide does not need to contain Al nor a rare earth element other than Ce because Ce alone partially substituting for Zr of the zirconium complex oxide can restrain the grain growth of the complex oxide particles to some extent. This is why the ranges for the xe2x80x9cx+yxe2x80x9d and the xe2x80x9cxxe2x80x9d coincide. However, since the inclusion of Al and/or a rare earth element other than Ce better restrains the growth of the zirconium complex oxide particles, the value of the xe2x80x9cyxe2x80x9d should be preferably set in the range of 0.02xcx9c0.2. The value of the xe2x80x9cyxe2x80x9d in excess of 0.2 may give rise to side products other than the desired zirconium complex oxide.
The zirconium complex oxide particles may be preferably coated on the heat-resistant support together with the oxygen-storing complex oxide particles and particles of a heat-resistant inorganic oxide. In this case, it is particularly advantageous if the combination of Pt and Rh is selectively carried only on the zirconium complex oxide particles.
Further, the heat-resistant inorganic oxide may be preferably selected from a group consisting of alumina, silica, titania and magnesia all of which are commercially available. Particularly useful is activated alumina.
The oxygen-storing oxide of the rare earth metal may be preferably cerium oxide or a cerium complex oxide. Further, a precious metal such as Pd may be selectively carried only on the particles of the oxygen-storing oxide in addition to the combination of Pt and Rh selectively carried only on the particles of the zirconium complex oxide.
The heat-resistant support, which may be made of cordierite, mullite, xcex1-alumina or a metal (e.g. stainless steel), should preferably have a honeycomb structure. In producing the catalytic converter, 10xcx9c200 g of the zirconium complex oxide with a specific surface area of 50xcx9c160 m2/g, 30xcx9c150 g of the oxygen-storing oxide with a specific surface area of 100xcx9c200 m2/g and 0xcx9c200 g of the heat-resistant inorganic oxide with a specific surface area of 150xcx9c200 m2/g may be coated together, by the known wash-coating method, over the honeycomb support per dm3 (apparent volume) thereof.
The particles of the zirconium complex oxide may preferably have an average grain size of 0.1xcx9c2 xcexcm. In a typical example, 0.2xcx9c2 g of Pt and 0.04xcx9c1 g of Rh may be supported on the particles of the zirconium complex oxide per dm3 (apparent volume) of the honeycomb support.
The zirconium complex oxide according to the present invention may be prepared by using known techniques such as coprecipitation process or alkoxide process.
The coprecipitation process includes the steps of preparing a solution which contains respective salts of zirconium, cerium and optionally Al and/or a rare earth element other than cerium in a predetermined stoichiometric ratio, then adding an aqueous alkaline solution or an organic acid to the salt solution for causing the respective salts to coprecipitate, and thereafter heat-treating the resulting coprecipitate for oxidization to provide a target zirconium complex oxide.
Examples of zirconium salts include zirconium oxychloride, zirconium oxynitrate, zirconium oxysulfate and zirconium oxyacetate. Examples of salts of cerium and other rare earth elements (and/or Al) include sulfates, nitrates, hydrochlorides, phosphates, acetates and oxalates. Examples of aqueous alkaline solutions include an aqueous solution of sodium carbonate, aqueous ammonia and an aqueous solution of ammonium carbonate. Examples of organic acids include oxalic acid and citric acid.
The heat treatment in the coprecipitation process includes a heat-drying step for drying the coprecipitate at about 50xcx9c200xc2x0 C. for about 1xcx9c48 hours after filtration, and a baking step for baking the coprecipitate at about 350xcx9c1,000xc2x0 C. (preferably about 400xcx9c700xc2x0 C.) for about 1xcx9c12hours. During the baking step, the baking conditions (the baking temperature and the baking period) should be selected depending on the composition of the zirconium complex oxide so that at least part of the zirconium complex oxide is in the form of solid solution.
The alkoxide process includes the steps of preparing an alkoxide mixture solution which contains zirconium, cerium and optionally Al and/or a rare earth element other than cerium in a predetermined stoichiometric ratio, then adding a deionized water to the alkoxide mixture solution for causing zirconium, cerium and Al (and/or rare earth element other than Ce) to coprecipitate or hydrolyze, and thereafter heat-treating the resulting coprecipitate or hydrolysate to provide a target zirconium complex oxide.
Examples of alkoxides usable for preparing the alkoxide mixture solution include respective methoxides, ethoxides, propoxides and butoxides of zirconium, cerium, and Al (and/or rare earth element other than Ce). Instead, ethylene oxide addition salts of each of these elements are also usable.
The heat treatment in the alkoxide process may be performed in the same way as that in the coprecipitation process.
Pt and Rh may be supported on the zirconium complex oxide by using known techniques. For instance, a solution containing a respective salt (e.g. 1xcx9c20 wt %) of Pt and Rh is first prepared, the zirconium complex oxide is then impregnated with the salt-containing solution, and thereafter the zirconium complex oxide is heat-treated. Examples of salts usable for this purpose include nitrate, dinitro diammine nitrate, and chloride. The heat-treatment, which is performed after impregnation and filtration, may include drying the zirconiumcomplex oxide by heating at about 50xcx9c200xc2x0 C. for about 1xcx9c48 hours and thereafter baking the complex oxide at about 350xcx9c1,000xc2x0 C. for about 1xcx9c12 hours.
Other features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments given with reference to the accompanying drawings.